Alternating-current induction motors have starting currents which are several times larger than their full-load running currents. This high starting current is a disadvantage, requiring large copper conductors in the power system distribution lines, and requiring distribution transformers sized for the large currents. Examples of motors to which this invention is applied and of single-phase motors which have this starting characteristic are given in U.S. Pat. No. 4,792,740, issued Dec. 20, 1988 to Otto J. M. Smith, entitled "Three-Phase Induction Motor With Single-Phase Power Supply." This invention is an improved method of starting these motors. Further examples of induction motors to which this invention applies are given in U.S. Pat. No. 5,300,870, issued Apr. 5, 1994, entitled "Three-Phase Motor Control," and in U.S. Pat. No. 5,545,965, issued Aug. 13, 1996, entitled "Three-Phase Motor Operated From A Single-Phase Power Supply And Phase Converter," both issued to Otto J. M. Smith. The winding circuit of the '870 patent is called a Semihex.TM. configuration. Here, the 3 windings have 120.degree. phase differences but are series connected (as compared to wye or delta).
The starting currents of induction motors have poor power-factors, which is undesirable because the power supply company must provide a large starting current, although the wattage for which the company is paid is small.
The full-load currents of induction motors have poor power-factors, which is undesirable because the power supply company must provide a large full-load current, although the wattage for which the company is paid is smaller.
Some loads which must be started gently require reduced-torque starting, and new methods for doing so are disclosed herein.
Three-phase wye-start delta-run motors are common for large motors. The wye-start circuit reduces the voltage per winding, reduces the magnetic flux in the air gap, reduces the starting current, and reduces the shaft starting torque.